Authorities are urging schools to close and invoking restrictions on factories and traffic that will keep half the city's vehicles off the roads.

The red alert - the most serious warning on a four-tier system adopted in recent years - was announced late on Monday. It means authorities have forecast more than three consecutive days of severe smog.

Readings of PM2.5 particles climbed toward 300 micrograms per cubic meter, compared with the World Health Organisation's safe level of 25. The heavy smog is not expected to improve until Thursday.

A man walks on a bridge on a hazy day in Beijing, China, December 7, 2015. REUTERS/Jason Lee

It is the second time this month that notoriously polluted Beijing has experienced a prolonged bout of smog.

Most of the pollution is blamed on coal-fired power plants, along with vehicle emissions and construction and factory work.